Materials of easy-opening can end mainly adopt aluminum. Coated steel sheets have a small economical advantage because they need a repair-coating step after forming the can end, though they are inexpensive as the base materials of the can end. Consequently, steel sheets are not positively used in the field. With the background, various trials were given to eliminate the repair-coating step of easy-opening can end fabricated by steel sheet applying a new end-panel working method and using a laminated steel sheet for the method.
As of these trials, JP-A-6-115546, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), tries to eliminate the repair step by improving the working method for forming the conventional V-shape score that has a flat part at bottom of the score, using a polyester resin. JP-A-9-234534 improves the score-working method while specifying the thickness and the breaking elongation of resin layer. JP-A-11-91775 tries the elimination of repair step using a curved surface mold for score-working.
In spite of these prior technologies, the market is occupied by the aluminum-made end panels. With the above increasing trend for easy-opening can, the easy-opening can end made of aluminum has increased not only in the beverage can market but also in the food can market. The phenomenon shows that the market does not evaluate the merit of switching from the aluminum can end to the laminated steel sheet can end.
The market is reluctant to accept the laminated steel sheet can end is presumably that many of them are inapplicable depending on the required level of can design, can end-panel forming method, can-manufacturing method, and the like, though some of them satisfy these requirements. That is, there exist end panel types and end-panel working conditions that hinder the elimination of repair step for steel end panels. Specific cases of raising the difficulties include an end-panel design requiring further severe score-working to decrease the can-opening force and a temperature-increase in the mold for successively fabricating the end panels affecting the resin working.
Consequently, to eliminate the repair step for almost all kinds of can ends, there are required to establish a working method and a resin that do not induce break of the resin layer even under those severe working conditions.
The conventional score-working method, however, cannot fully eliminate the repair step under those severer score-working conditions. For example, according to a survey given by the inventors of the present invention, the score-working technology disclosed in JP-A-11-91775 is the most promising one in the related art. The disclosure describes score-working using a mold having a curved surface with a constant curvature. That is, the disclosure obtained the residual thickness of the score in a range from 0.025 to 0.080 mm at a radius of curvature of the mold in a range from 0.1 to 1.0 mm. According the disclosure, the workability drastically improved compared with the sharp-notch type in the related art. However, the disclosed technology failed to fully satisfy the above-described severe score-working conditions.
The full scale introduction of the laminated steel sheets to the market is achieved if the above problems are solved. The full scale introduction of the inexpensive laminated steel sheets to the market not merely decreases the can cost. That is, since the all-steel cans are superior in the recycle point of view and since the steel base material is the one applying lighter load to environment compared with aluminum, the shift of base material has significance also in the industries.